Not Applicable.
Not Applicable.
The present invention relates generally to a welding power supply having an improved isolation circuit. More specifically, the present invention relates to a system for isolating a control circuit from a switch in a welding power supply.
Welding power supplies are typically stand-alone units which receive a standard line voltage and provide a usable welding power at a welding output. The welding power may be alternating current (AC) or direct current (DC), continuous current or constant voltage, three-phase or single-phase, and may include a wide range of amperages, all depending upon operator-selected inputs. Various power and control circuitry is used to shape and time the welding power based upon the operator-selected inputs.
Many welding power supplies utilize switches or gating devices, such as silicon-controlled rectifiers (SCRs), to control the amount of power provided at the welding output. An SCR is a three-terminal device which provides current from an anode to a cathode in response to a current provided to a gate. SCRs are in wide usage in welding power supplies. A control circuit is used to drive the gate to control the SCR.
To drive multiple SCRs in a power conversion circuit, it is necessary to isolate the control circuit from the SCR. A conventional isolation circuit 10 is illustrated in FIG. 1. A command signal is received on the gate 12 of a transistor 14 when it is time to gate the SCR. Transistor 14 turns on and induces a current in a transformer 16. The command signal must be discontinued before transformer 16 saturates. The current created in the primary coil of transistor 16 is reflected on the secondary coil as a gate drive signal, where it is fed through a diode 18 to the gate 20 of SCR 22. Diode 18 protects the secondary coil of transformer 16 from reverse current generated by SCR 22 after it has turned on. The command signal cannot be repeated until transformer 16 resets. The command signal may be reapplied after waiting a sufficient time period for transformer 16 to reset. However, if the command signal is reapplied too soon, the drive current to SCR 22 will be reduced, and it may not be high enough to gate SCR 22. Thus, it would be advantageous if the gate drive signal could be continuous in order to ensure that SCR 22 turns on and remains on so long as the command signal is received.
In addition, transformer 16 must have a high enough primary inductance to sustain a gate drive-signal long enough to fire SCR 22. A typical inductance value is on the order of 20 mH, which requires a large pulse transformer.
One alternative to the circuit of FIG. 1 is to use an opto-isolator circuit. An opto-isolator circuit feeds current from an anode side of the SCR through the opto-switch into the gate. This can become problematic because the voltage source feeding the SCR is an AC signal which is always changing. Thus, the drive current used to turn on the SCR is inconsistent.
Accordingly, there is a need for an improved isolation circuit for a welding power supply. Further, there is a need for an isolation circuit which can provide a continuous current drive to an SCR gate. Further still, there is a need for an isolation circuit having a smaller primary inductance, allowing a smaller, lower-cost transformer to be used. Further yet, there is a need for an isolation circuit which provides a continuous current drive to a switch or gating device in response to a simple digital command signal. The teachings hereinbelow extend to those embodiments which fall within the scope of the appended claims, regardless of whether they accomplish one or more of the above needs.
According to a first exemplary embodiment, a circuit for controlling the welding power of a welding power supply includes a control circuit, a switch, and an isolation circuit. The control circuit is configured to generate a command signal. The isolation circuit has a flyback transformer and is configured to receive the command signal and to provide a switch drive signal to the switch in response to the command signal. The switch provides welding power in response to the switch drive signal.
According to another exemplary embodiment, a welding power supply includes a power conversion circuit, a control circuit, and an isolation circuit. The power conversion circuit has at least one switch configured to provide welding power. The control circuit is configured to generate a command signal. The isolation circuit is configured to provide isolation between the switch and the control circuit. The isolation circuit is responsive to the command signal to charge a coil during a first phase and discharge the coil during a second phase. The coil provides current to the switch only during the second phase.
According to yet another exemplary embodiment, a circuit for controlling the welding power of a welding power supply includes a means for generating a command signal, a means for gating welding power from a power source to a welding output, and a means for isolating the means for generating from the means for gating and for providing a continuous current drive signal to the means for gating.